Phenoxyalkanoic herbicides

Phenoxyalkanoics. The phenoxyalkanoic herbicide grouping is composed of two subgroups, the phenoxyacetic acids and the phenoxypropionic acids. The phenoxyacetic acid herbicides include some of the first commercially successhil herbicides, eg, 2,4-D. They continue to be widely used for foUar control of broadleaf weeds. The more heavily functionalized phenoxypropionic acid herbicides are relatively new herbicides compared to the phenoxyacetic acids and are used primarily for selective control of grassy weeds in broadleaf crops (2,296,297). 

The phenoxyalkanoic herbicides are acidic in nature and thus subject to some degree of ionization. The extent to which the herbicide ionizes is controlled by the acid dissociation constant (fQ of the herbicide in question and the soil solution pH (238). The leaching potential is significantly influenced by these reactions. 

Another factor that can influence the environmental distribution of a chemical is the presence of charged groups. Some pollutants, such as the sodium or potassium salts of phenoxyalkanoic herbicides, dinitrophenols, and tetra- or penta-chlo-rophenol, exist as anions in solution. Others, such as the bipyridyl herbicides diquat and paraquat, are present as cations. In either case, the ions may become bound to organic macromolecules or minerals of soils or sediments that bear the opposite... 

PCDDs have been released into the environment in a number of different ways. Sometimes this has been due to the use of a pesticide that is contaminated with them. 2,4,5-T and related phenoxyalkanoic herbicides have been contaminated with them as a consequence of the interaction of chlorophenols used in the manufacturing... 

PCDFs are similar in many respects to PCDDs but have been less well studied, and will be mentioned only briefly here. Their chemical structure is shown in Figure 7.1. Like PCDDs, they can be formed by the interaction of chlorophenols, and are found in commercial preparations of chlorinated phenols and in products derived from phenols (e.g., 2,4,5-T and related phenoxyalkanoic herbicides). They are also present in commercial polychlorinated biphenyl (PCB) mixtures, and can be formed... 

Schneiderheinze, J.M., Armstrong, D.W., Berthod, A. (1999). Plant and soil enantioselective biodegradation of racemic phenoxyalkanoic herbicides. Chirality 11, 330-337. 

Phenox process, 10 787 Phenoxyacetic acid herbicides, 13 314 Phenoxyacetic acids, 13 50, 304 Phenoxyalkanoic acids, 13 284 Phenoxyalkanoic herbicides,... 

Que Hee SS, Sutherland RG (1981) Chemistry, analysis, and environmental pollution, the phenoxyalkanoic herbicides, vol 1. In CRC Series in Pesticide Chemistry. CRC Press, Boca Raton... 

Hee, S. S. Q. Sutherland, R. G. "The Phenoxyalkanoic Herbicides, Volume I, Chemistry, Analysis, and Environmental Pollution," CRC Press, Inc., Boca Raton, Florida, 1981. 

It must be remembered, however, that laboratory studies with uniform herbicide applications are not equivalent to field applications where the herbicide initially comes into contact with the soil surface. A herbicide applied to the soil surface at a rate of 1.0 kg/ha is roughly equivalent to 2 ppm assuming distribution in the top 5 cm (23). This would be equivalent to 4 ppm if distributed in the top 2.5 cm of field soil and 32 ppm if contained in the surface 0.32 cm. Thus, under field conditions, concentrations of phenoxyalkanoic herbicides at the soil surface at the time of application could be considerably higher than those used in some laboratory studies, and biphasic breakdown may be possible. 

There is evidence from a relatively few studies to show that repeated treatments of 2,4-D and MCPA result in enhanced degradation rates under field conditions as a result of adaptation of soil microorganisms. Even less is known about the phenomenon of cross-enhancement under field conditions, where previous applications of either 2,4-D and MCPA may result in enhanced breakdown of both phenoxyalkanoic herbicides. 

Many aromatic organics, such as phenoxyalkanoate herbicides, contain fatty acid side chains that can be metabolized by (3-oxidation. (3-Oxidation proceeds by the stepwise cleavage of two-carbon fragments from a fatty acid. The short-chain fatty acid is further decomposed until the chain length is four or two carbons. 

D and related phenoxyalkanoic herbicides are highly effective against broad leafed plants and have found application under a wide variety of situations m weed control in lawns, gardens, cereal crops, and pastures to defoliants in forestry and warfore. In the early 1980 s, production of 2,4-D in USA alone was approximately 13 million pounds per year. The fact that large quantities of 2,4-D and its related herbicides are manufactiued and applied each year, necessitates the impmtance of... 

Triazines, amides, phenylureas, triazoles, triazinones, benzimidazoles, phenoxyalkanoic herbicides LC/MS and LC/MS/MS ESI (-) for acidic compounds APCI ( + ) 20-100 pgmr 2-6pgmr Environmental water... 

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